The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the inventors hereof, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
The disclosed technology relates to communications systems, and more particularly, to stopping criteria for determining when to terminate iterative interference cancellation techniques.
In a data transmission system, it is desirable for information, often grouped into packets, to be accurately received at a destination. A transmitter at or near the source sends the information provided by the source via a signal or signal vector. A receiver at or near the destination processes the signal sent by the transmitter. The medium, or media, between the transmitter and receiver, through which the information is sent, may corrupt the signal such that the receiver is unable to correctly reconstruct the transmitted information. Therefore, given a transmission medium, sufficient reliability is obtained through careful design of the transmitter and/or receiver, and of their respective components.
Interference cancellation is a technique for improving the performance of a data transmission system. According to this technique, when decoding a plurality of codewords that interfere with one another, effects of partially decoded codewords are subtracted from a received signal vector to reduce or eliminate interference of the partially decoded codeword on the remaining codewords. For example, a received codeword that is associated with a small amount of corruption (e.g., due to favorable channel conditions or more robust encoding) may be decoded before other codewords that are associated with larger amounts of corruption. The codeword that is decoded first may then be subtracted to reduce or eliminate its interference on the codewords associated with the larger degree of corruption. This, in turn, facilitates the decoding of these codewords. In this way, the other codewords may experience less interference and are able to achieve a higher signal-to-interference-plus-noise ratio (SINR) than without interference cancellation.
Interference cancellation may be performed in a parallel or serial fashion. Serial interference cancellation schemes may also be referred to as successive interference cancellation techniques. Parallel interference cancellation techniques may process all of the received codewords concurrently to obtain “soft” decoding metrics that associate a likelihood of a decoding hypothesis with each bit in the codeword, such as the likelihood that a certain bit is a logical one rather than a logical zero. These likelihoods may be expressed in various ways such as, for example, log-likelihood ratios (LLRs). Parallel interference cancellation techniques may update the aforementioned likelihoods of a given codeword based on the likelihoods of other codewords. For example, if a first codeword has a high likelihood of having been correctly decoded, the interference of this codeword on other codewords may be subtracted out based on the preliminary decoding hypotheses of the first codeword. Parallel interference cancellation techniques are called “parallel” because codewords are processed at the same time, and for each codeword, the interference from many or all other codewords may be partially removed based on the preliminary decoding hypotheses for these codewords.
In contrast, for successive interference cancellation techniques, codewords are decoded in order. For example, a codeword that has a small degree of corruption may be decoded first, because it is most likely to result in an accurate decoding hypothesis. The impact of this first codeword is then subtracted from the received signal. Next, the second codeword may be decoded, and its impact may be subtracted from the received signal. This process continues until all codewords have been decoded. This technique is referred to as “serial” or “successive” interference cancellation because codewords are decoded one after the other.
The order in which codewords are decoded is an important design consideration. Successive interference cancellation methods may decode codewords in an order that is based on information that reflects long-term channel conditions or channel performance, such as a long-term signal-to-interference-plus-noise ratio. Successive interference cancellation technique may also take into account channel and interference conditions pertaining to a specific transmission attempt (e.g., in a specific subframe of the communication system).